CLASSIFICATION OF STEUCTURAL STEELS. 399
tain heat is to he rolled into angles, or plates, or eye-bars, and it is seldom necessary to put part of a heat into one shape and part into another. On the other hand, it is almost always necessary to roll a charge into more than one thickness and more than one size of angles, plates, etc., and it is an onerous restriction if proper allowance be not made for the variations due to different thickness.
SEC. XYIIIe.—Standard size of test-pieces.—In all the tensile tests a length of eight inches should be taken as the standard for all sections. For several years there have been conferences held in foreign lands to establish uniform methods of testing, and it has been officially recommended that in the case of rounds the length of the test-piece shall be proportional to the square root of the sectional area, the formula being given as follows: 1=12.0 V / when Z=the length in inches and /=the sectional area in square inches, In Table XVIII-B I have calculated from this formula the proper length for rounds from one-half inch to 1|- inches in diameter. The length is greatly reduced as the diameter grows less, and this is equivalent to demanding less elongation, while on larger sizes the length is increased, this being the same thing as demanding more elongation.
It is difficult to compare this system, in which the elongation is constant and the length varies, with the system wherein the length is constant and the required elongation varies; but an attempt is made to do this by obtaining the proportional elongation for the different lengths from Curve A A in Fig. XVI-A, the results being given in the last column of the table. A long time has been spent in arriving at the standard length of eight inches, and it would be very unfortunate if a complicated substitute were introduced. Such a change, however, is unlikely from present indications.
It is understood throughout this book that the elastic limit is determined by the drop of the beam. T have no sympathy with that group of agitators who are trying to introduce new meanings to old terms, and to apply old terms to new factors. It matters not whether the drop of the beam does or does not mark the spot where the elongation ceases to be exactly proportionate to the load. It represents a critical point of failure, and this is acknowledged by the agitators before mentioned, who recommend its determination on all test-pieces.
Moreover, it is shown in Section XVIm that this is a definite